The invention concerns a marine structure, a method of installing a marine structure and a suction pile. Particularly, the invention concerns application to a so called xe2x80x9cminimal platformxe2x80x9d.
Suction piles and their way of installing are, for example, known from GB-B-2300661 and EP-B-0011894, which disclosures are incorporated herein by reference. Briefly, a suction pile is a thin walled steel cylinder, closed at at least one longitudinal end, that is located on the subsea bottom with the opposite end and penetrates the subsea bottom with the aid of a suction created within the cylinder. The creation of the suction can be with the aid of a suction source, such as a pump, being on, or close to or at a distance (e.g. above the water surface, e.g. at a vessel) from the suction pile. The applied level of the suction can be e.g. at least substantially constant, smoothly increase or decrease or else pulsate, for which there are convenient means; for an e.g. pulsating level a possibly in the suction pile integrated pressure accumulator that is intermittently connected to the inner space of the cylinder.
After use, the suction pile can easily be removed by creating an overpressure within the cylinder, e.g. by pumping in (sea)water.
A self installing platform applying suction piles which provide buoyancy is known from WO99/51821 of the present inventor.
Instead of installing a suction pile into the under water bottom by generating a fluid pressure difference between the inside and outside of the suction pile, it is also feasible that the suction pile at least partly penetrates the under water bottom by a weight resting on it, e.g. the platform and/or a ballast body.
Therefor according to the invention it is proposed to make the marine structure self floating and self foundating/installing by providing it with buoyancy and one or more suction piles. So the hoisting device and the foundation plant can be eliminated. Preferably the structure has buoyancy of its own, e.g. obtained by the with the structure integrated appliance that is designed to, once the structure is installed, ballast the structure. Buoyancy can also be obtained from the suction pile, which for that can be provided with a floater. Said own buoyancy is preferably such that it is substantially contributing to the required buoyancy to make the structure self floating. It is preferable, if the buoyancy can be at least substantially decreased for installation purposes. By e.g. filling the one or more floating bodies with ballast, like water. Therefor it is convenient, to provide the structure with means for admitting and possibly removing of ballast, such as between the closed and open position switchable shutter valves in a water supplying respectively water venting opening to a ballast tank.
Since the structure is self floating and is provided with one or more suction piles, removal after use is made easier. On the one hand in that by pressing out the suction pile, the anchoring of the structure to the underwater bottom can be removed. On the other hand in that the structure can independently rise to the water surface by the (possibly regained) buoyancy.
In this respect the marine structure typically will be relatively small, e.g. a production platform with equipment. Due to its own weight, such marine structure is designed to be applied with a foundation of pile bodies to be pressed in the bottom. The marine structure preferably has, apart from the suction piles, no floating bodies, apart from parasitic floating bodies such as air filled spaces that are normally present, such as frame tubes. The marine structure according the the present invention will typically weigh not more than about 50,000 kilo, although structures with a much higher weight of e.g. 300,000 kilo or more are also feasible.
For the purpose of transporting to the final destination it is prefered that the platform rests on a barge or other separate floating and/or sailing body. Preferably said barge has insufficient buoyancy of its own to keep the marine structure floating, considering the prevailing safety requirements. The one or more suction piles provide the additionally required buoyancy. xe2x80x9cBargexe2x80x9d means at least a vessel known as such with one or more floating spaces at least substantially hermetically delimited from the environment. Preferably the barge has no equipment of its own for propelling and/or directional control.
Thus a cost and time and energy and environmental saving is possible, and also the work is safer.
To provide the additional buoyancy, the suction pile is preferably provided with a floating means. The floating means can at least substantially comprise a space in open communication with the surrounding water at its under side, such as the pressure space of the suction pile, e.g. if the suction pile will float in the water at least as much upright as possible. If one can keep said space free of water to a satisfying level, the desired buoyancy can be maintained without requiring to delimit this floating space at all sides with respect to the water environment. Said space can therefor e.g. be connected to a convenient means, such as for delivering a gas generating dry compound into said space, or for delivering into said space a pressurised gas, such as a pump, to generate a convenient gas pressure in the suction space and to possibly maintain it against the pressure of the surrounding water. Due to the movements of the floating suction pile in the water, it is expected that without counter measurements this space will be filled more and more with water from below. A remedy is to continuously or intermittendly removing of the incoming water by e.g. refilling said space with gas, for which said above mentioned means is/are continuously or intermittendly activated. In this connection it is preferred to integrate this means in an active, preferably automatic, e.g. electronic control circuit wherein said means is activated in dependency from the detection of the buoyancy of the suction pile at different times, such as by measuring e.g. the water surface or e.g. the gas pressure within said space with e.g. a convenient sensor, outputting its measuring signal to an evaluation device comparing the measuring signal with an input value, switching on or letting switching on said means to get back to the initial situation once a treshold difference value is exceeded.
Application of the above described space in open communication with its surrounding water has drawbacks in view of ensuring the buoyancy. It is therefor preferable if said floating means provides one or more floating spaces that are delimited at all sides with respect to the water environment and that are filled with a floating substance, such as air or a gas or some other material of relatively low specific weight. Said floating means can comprise e.g. a seperate, inflatable, completely closed, diafragm type floating body, preferably within the suction pile, e.g. in the suction space. With e.g. a space of the suction pile that is open at its lower side, use can be made of an airtight bulkhead with which said opening can be sealed. If said bulkhead is at least substantially rigid, e.g. of metal, preferably steel, of sufficient thickness, it can withstand a pressure difference between said space and its environment by bearing bending stresses, hoop stresses or a combination of both. Then it is for realising and maintaining the desired buoyancy not necessary to bring this space to a pressure that is substantially higher than atmospheric pressure. If the bulk-head is substantially flexible, e.g. as an elastic or plastic excellent deformable membrane of e.g. rubber, it can be necessary for obtaining and maintaining sufficient buoyancy to bring this space to a pressure substantially higher than atmospheric pressure.
To be able to obtain the desired pressure within the pressure space it is prefered that said space is hermetically limited. It is prefered to connect the pressure space with a convenient means to supply said space with pressurised gas.
The meaning of xe2x80x9cdelimited at all sides with respect to the water environmentxe2x80x9d here is that a boundary with respect to the surrounding air is not required. The meaning of xe2x80x9chermetically delimitedxe2x80x9d here is a boundary both with respect to the surrounding water and the surrounding air.
The invention is also concerned with a method for transporting a marine structure over water making use of one or more suction piles with buoyancy, providing at least substantially all buoyancy of said marine structure.